Ensuring register availability for dynamic binary optimization

ABSTRACT

A compiler compiles code in a target program by reserving at least one register for use by a dynamic binary optimizer during target program execution. When the target program is subsequently executed, the dynamic binary optimizer stores needed state information in the reserved register(s), without affecting register state of the target program. Preferably, the state information in the reserved register(s) includes addressing information for a context save area, used for saving processor state when switching context from the target program to the dynamic binary optimizer.

FIELD OF THE INVENTION

The present invention relates to digital data processing, and inparticular to dynamic translation, compilation and optimization ofcomputer programming code.

BACKGROUND

In the latter half of the twentieth century, there began a phenomenonknown as the information revolution. While the information revolution isa historical development broader in scope than any one event or machine,no single device has come to represent the information revolution morethan the digital electronic computer. The development of computersystems has surely been a revolution. Each year, computer systems growfaster, store more data, and provide more applications to their users.

At the heart of a computer system is one or more central processingunits (CPUs), also called processors, which execute instructions storedin the computer's memory. From the standpoint of the computer'shardware, most systems operate in fundamentally the same manner.Processors are capable of performing a defined set of very simpleoperations, such as arithmetic, logical comparisons, and movement ofdata from one location to another. But each operation is performed veryquickly. Computer program code at multiple levels directs the computerto perform massive numbers of these simple operations, enabling thecomputer to perform complex tasks. Although the defined set of simpleoperations is limited, the possible sequences and combinations of suchoperations which can be specified in the program code are virtuallylimitless.

In the very early history of the digital computer, computer programswhich instructed the computer to perform some task were written in aform directly executable by the computer's processor. Such programs werevery difficult for a human to write, understand and maintain, even whenperforming relatively simple tasks. As the number and complexity of suchprograms grew, this method became clearly unworkable. To make computerprograms easier to develop, a large and varied set of high-levellanguages was developed for supporting the creation of computer programcode.

High-level languages vary in their characteristics, but all suchlanguages are intended to make it easier for a human to write a programto perform some task. Typically, high-level languages representoperations, fixed values, variables, and other constructs in a mannerreadily understandable to the human programmer rather than the computer.Such programs are not directly executable by the computer's processor.In order to run on the computer, the programs must first be transformedfrom a human-readable form (source code) to something executable by theprocessor of a computer, i.e. to a sequence of instructions directlyreadable and executable by the processor.

An instruction which is directly readable and executable by theprocessor (a processor-executable instruction) is a sequence of binarybits in a pre-defined format, each bit position being specific to logicwithin the processor which reads and decodes it. Combinations of bitvalues specify an operation or operations to be performed, source ordestination of data, branch conditions or destinations, and so forth.The bit sequence formatting and the meaning of the bit combinationsdefines the “instruction set” of the processor. While the limited set ofsimple operations performed by any processor is similar to that ofanother, the instruction set of each processor (i.e., the pre-definedformat and meaning of the processor's executable instruction binary bitsequence) is not.

In general, source code is universal and understandable by anyonetrained to use the applicable language, while executable code isspecific to a particular computer system environment, and can onlyexecute on that computer system or one similarly configured. Inparticular, the executable code is specific to the processor'sinstruction set, although it may be specific to other parameters of thecomputer system as well.

Various techniques exist for transforming the source code in ahigh-level language to processor-executable instructions in theprocessor's instruction set. Source code can be “interpreted”, meaningthat a special program (an “interpreter”) takes each source codestatement in sequence, and executes a small procedure (i.e., series ofinstructions in the processor-executable instruction set) correspondingto each source code instruction. Interpreting is useful for somepurposes, but it is generally rather inefficient.

Traditionally, for greater execution efficiency, individual portions(modules) of source code are compiled to form modules ofprocessor-executable instructions, which may be linked together to formlarger programs (although some programs contain only a single compiledmodule). These programs are saved in digital media storage in executableform, and may be distributed in that form to other computer systems, orremain on the system in which they were first compiled. In this formthey are later executed, usually many times. Compilation is itself atask performed by a special computer program (a compiler), and can takesignificant time. Often, compilation involves certain optimizations tothe executable code which require analysis of the various instructionsequences within the program. Unlike interpreted code, the resultantprocessor-executable instructions in a compiled module do notnecessarily correspond to particular source instructions, and do notnecessarily follow the same sequence, although they must produce thesame logical output. Since it is expected that such programs will beexecuted many times, the burden of compilation is spread over manyexecutions. This traditional form of compilation is sometimes referredto as “static compilation”.

In recent years, there has been increasing interest in “just-in-time”dynamic compilation or optimization. Like static compilation,“just-in-time” or dynamic compilation/optimization involves thegeneration of optimized processor-executable instructions. But unlikestatic compilation, the program's processor-executable instructions aregenerated during or as part of execution of the program of which theprocessor-executable instructions are a part (the “target program”).This effectively means that just-in-time or dynamiccompilation/optimization is intended to be performed many times, forexample, each time the program is executed, or for each user processwhich executes the program.

Obviously, when compared with traditional static compilation, dynamiccompilation/optimization suffers from the drawback that it is performedagain each time the program is executed or for each new user process.However, there are various advantages to just-in-time or dynamiccompilation/optimization that make it attractive in many circumstances.For example, dynamically compiled code is generated on the system thatwill execute it, and can be generated from a universal, intermediatelevel code form (between a high-level source language and aprocessor-executable form), which facilitates portability of theprogram. Such an approach is frequently used in the well-known JAVA™virtual machine environment. Furthermore, additional knowledge availableto the system at execution time, such as the exact system configuration,the code modules being used, and the actual pattern of code execution,make it possible to generate more efficient compiled code than cantypically be generated using a static compiler. Such additionalknowledge is particularly effective in the case of actual code usagepatterns, which allow dynamic compilation/optimization to focus onoptimizing specific portions of the code.

One form of dynamic compilation is the generation ofprocessor-executable instructions at execution time from previouslycompiled executable code, generally without use of the original sourcecode. This is known as dynamic binary optimization. Dynamic binaryoptimization is commonly used to emulate a computer system having adifferent, often older, instruction set architecture. I.e., anexecutable computer program which was previously compiled to containprocessor-executable instructions in an older instruction setarchitecture can be recompiled from the previously compiled executablecode to generate processor-executable instructions in the instructionset of the current processor.

Although dynamic binary optimization is commonly used to supportemulation, it has the potential for improving execution efficiency evenwhere the originally compiled executable code is compiled to the sameinstruction set architecture as that of the current processor. In otherwords, even though the previously compiled version of the program isfully capable of execution on the current processor without furthercompilation, optimization or translation, the execution efficiencieswhich are possible through dynamic binary optimization may in somecircumstances justify the additional overhead of dynamicallyre-compiling the already compiled code. In some cases, these executionefficiencies may be magnified if the instruction set architecture hasbeen enhanced with new features not utilized by the original staticallycompiled code (which may be quite old). Typically, only selectiveportions of the previously compiled code will be re-compileddynamically, because one of the advantages of dynamic compilation isthat it can identify “hot spots” (frequently executed code portions) andfocus optimization activity on these hot spots.

One difficulty with dynamic binary optimization of a program using thesame instruction set architecture arises because the compiler isgenerally free to use all available registers in the applicableinstruction set architecture for program optimizations. Therefore thetarget program, when executed in its originally compiled form, oftenuses all the available registers. Dynamic binary optimization isperformed by another program executing on the same computer, called a“virtual machine”. The virtual machine must use these same registers, sothe register contents are saved to storage and restored from storagewhen switching execution contexts between the virtual machine and thetarget program. In order to save the registers in most computerarchitectures, at least one register must be available to hold addressinformation for the area in memory where the registers will be saved(the “context save area”). If the target program is compiled to use allavailable registers, there is no guarantee that a register for holdingaddress information for the context save area will be available.

Although it has been proposed to solve this problem by dynamicallylocating an unused register, and failing to find one, temporarilystoring (“spilling”) the contents of one or more actively used registersto memory, this solution raises various issues of strict register statecompatibility and data integrity/security. A need exists, notnecessarily recognized, for a better mechanism to support dynamic binaryoptimization, and particularly to support dynamic binary optimizationusing the same instruction set architecture as the target program.

SUMMARY

A compiler compiles code in a target program for later dynamic binaryoptimization by reserving at least one register for use by a dynamicbinary optimizer in holding state information during target programexecution. I.e., the compiler may use all available registers for targetprogram execution except the register(s) reserved for the dynamic binaryoptimizer. When the program is subsequently executed, the dynamic binaryoptimizer stores needed state information in the reserved register(s),without affecting register state of the target program.

In the preferred embodiment, the state information stored in thereserved register(s) specifically includes addressing information for acontext save area, in which process state is saved when target programexecution is suspended so that the dynamic binary optimizer (a virtualmachine) can execute. The target program is compiled with a staticcompiler which includes an option to reserve one or more registers forlater use in dynamic binary optimization. If the option is selected, thestatic compiler compiles the code for the target program as if thenumber of available registers for program use is the actual number ofavailable registers, less the number of registers reserved for later usein dynamic binary optimization. This statically compiled code is savedand can later be executed multiple times, each execution instance beingoptimized separately by the dynamic binary optimizer.

Preferably, only a single register is reserved, because a singleregister is generally sufficient to contain the necessary addressinginformation for the context save area, and it is undesirable to reservemore registers than necessary because it decreases the number ofregisters available for target program execution, thereby making theprogram less efficient. Generally, the reservation of only a singleregister will have no more than a marginal effect on target programperformance. However, it would alternatively be possible to reserveadditional registers, which could be useful for other functions of thedynamic binary optimizer.

In one exemplary embodiment, a previously compiled legacy programincludes both an executable code version containing processor-executableinstructions and an intermediate level code version, although the sourcecode is not necessarily available. The legacy program is re-compiledfrom the intermediate level code version, reserving one or moreregisters for later use in dynamic binary optimization.

In another exemplary embodiment, a program is compiled from its sourcecode version, reserving one or more registers for later use in dynamicbinary optimization.

The reservation of one or more registers when generating the compiledcode for use in later dynamic binary optimization, in accordance withthe preferred embodiment described herein, provides a simple andeffective technique for supporting context switching by a dynamic binaryoptimizer with only minimal effect on target program performance.

The details of the present invention, both as to its structure andoperation, can best be understood in reference to the accompanyingdrawings, in which like reference numerals refer to like parts, and inwhich:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a high-level block diagram of the major hardware components ofa computer system for use in dynamic binary optimization of computerprogram code, according to the preferred embodiment of the presentinvention.

FIG. 2 is a high-level block diagram showing the major hardwarecomponents of a processor within the computer system of the preferredembodiment.

FIG. 3 is a conceptual illustration of the major software components inmemory of the computer system of the preferred embodiment.

FIG. 4. is a high-level flow diagram showing a general process ofcreating an executable target program and executing the target programusing a dynamic binary optimizer, in accordance with the preferredembodiment.

FIG. 5 is a high-level flow diagram showing a process of staticallycompiling one or more modules of a target program for execution using adynamic binary optimizer, in accordance with the preferred embodiment.

FIG. 6 is a high-level flow diagram showing a process executing a targetprogram using a dynamic binary optimizer, in accordance with thepreferred embodiment.

DETAILED DESCRIPTION

Referring to the Drawing, wherein like numbers denote like partsthroughout the several views, FIG. 1 is a high-level representation ofthe major hardware components of a computer system 100 for use indynamic binary optimization of computer program code, according to thepreferred embodiment of the present invention. Computer system 100includes at least one general-purpose programmable processor (CPU) 101which executes instructions of a pre-defined instruction set andprocesses data from main memory 102. Main memory 102 is preferably arandom access memory using any of various memory technologies, in whichdata is loaded from storage or otherwise for processing by CPU 101.

One or more communications buses 105 provide a data communication pathfor transferring data among CPU 101, main memory 102 and various I/Ointerface units 111-114, which may also be known as I/O processors(IOPs) or I/O adapters (IOAs). The I/O interface units supportcommunication with a variety of storage and I/O devices. For example,terminal interface unit 111 supports the attachment of one or more userterminals 121-124. Storage interface unit 112 supports the attachment ofone or more direct access storage devices (DASD) 125-127 (which aretypically rotating magnetic disk drive storage devices, although theycould alternatively be other devices, including arrays of disk drivesconfigured to appear as a single large storage device to a host). I/Odevice interface unit 113 supports the attachment of any of variousother types of I/O devices, such as printer 128 and fax machine 129, itbeing understood that other or additional types of I/O devices could beused. Network interface 114 supports a connection to one or moreexternal networks 130 (of which one is shown) for communication with oneor more other digital devices. Network 130 may be any of various localor wide area networks as are known in the art. For example, network 130may be an Ethernet local area network, or it may be the Internet.

It should be understood that FIG. 1 is intended to depict therepresentative major components of computer system 100 at a high level,that individual components may have greater complexity than representedin FIG. 1, that components other than or in addition to those shown inFIG. 1 may be present, that the number, type and configuration of suchcomponents may vary, and that a large computer system will typicallyhave more components than represented in FIG. 1. Several particularexamples of such additional complexity or additional variations aredisclosed herein, it being understood that these are by way of exampleonly and are not necessarily the only such variations.

Although only a single CPU 101 is shown for illustrative purposes inFIG. 1, computer system 100 may contain multiple CPUs, as is known inthe art. Although main memory 102 is shown in FIG. 1 as a singlemonolithic entity, memory 102 may in fact be distributed and/orhierarchical, as is known in the art. E.g., memory may exist in multiplelevels of caches, and these caches may be further divided by function,so that one cache holds instructions while another holds non-instructiondata which is used by the processor or processors. Memory may further bedistributed and associated with different CPUs or sets of CPUs, as isknown in any of various so-called non-uniform memory access (NUMA)computer architectures. Although communications buses 105 are shown inFIG. 1 as a single entity, in fact communication among various systemcomponents is typically accomplished through a complex hierarchy ofbuses, interfaces, and so forth, in which higher-speed paths are usedfor communications between CPU 101 and memory 102, and lower speed pathsare used for communications with I/O interface units 111-114. Buses 105may be arranged in any of various forms, such as point-to-point links inhierarchical, star or web configurations, multiple hierarchical buses,parallel and redundant paths, etc. For example, as is known in a NUMAarchitecture, communications paths may be arranged on a nodal basis.Buses may use, e.g., an industry standard PCI bus, or any otherappropriate bus technology. While multiple I/O interface units are shownthat separate buses 105 from various communications paths running to thevarious I/O devices, it would alternatively be possible to connect someor all of the I/O devices directly to one or more system buses.

Computer system 100 depicted in FIG. 1 has multiple attached terminals121-124, such as might be typical of a multi-user “mainframe” computersystem. Typically, in such a case the actual number of attached devicesis greater than those shown in FIG. 1, although the present invention isnot limited to systems of any particular size. User workstations orterminals which access computer system 100 might be used to provide auser interface to users executing applications which access a database(similar to client applications which invoke a server to access adatabase over a network), but which execute directly on computer system100, without the need to communicate through a network. System 100 mayalternatively be a single-user system, typically containing only asingle user display and keyboard input. Furthermore, while the inventionherein is described for illustrative purposes as embodied in a singlecomputer system, the present invention could alternatively beimplemented using a distributed network of computer systems incommunication with one another, in which different functions or stepsdescribed herein are performed on different computer systems. Inparticular, static code compilation of a target program could beperformed on a first computer system, while execution of the same targetprogram and dynamic binary optimization during execution could beperformed on a second computer system.

While various system components have been described and shown at a highlevel, it should be understood that a typical computer system containsmany other components not shown, which are not essential to anunderstanding of the present invention. In the preferred embodiment,computer system 100 is a multi-user computer system, such as a computersystem based on the IBM i/Series™ architecture, it being understood thatthe present invention could be implemented on other computer systems.

FIG. 2 is a high-level diagram of the major components of a centralprocessor unit (CPU) 101, also sometimes called a processor or processorcore, including certain associated cache structures, according to thepreferred embodiment, showing CPU 101 in greater detail than is depictedin FIG. 1. As explained previously, system 100 may contain multipleCPUs, although only a single one is shown in FIG. 2. CPU 101 includesinstruction unit portion 201, special register portion 205, andexecution unit portion 211. Also shown in FIG. 2 are Level 1 InstructionCache (L1 I-Cache) 221, Level 1 Data Cache (L1 D-Cache) 222, Level 2Cache (L2 Cache) 223, Address Translation unit 224, and Memory Interface225. In general, instruction unit 201 obtains instructions from L1I-cache 221, decodes instructions of the CPU's pre-defined instructionset to determine operations to perform, and resolves branch conditionsto control program flow. Execution unit 211 performs arithmetic andlogical operations on data in registers, and loads or stores data fromL1 D-Cache 222. Special registers 205 contain various state data forcontrolling instruction flow and proper operation of the CPU notincluded in instruction unit 201 or execution unit 211. L2 Cache 223 isa level 2 cache, generally larger than L1 I-Cache 221 or L1 D-Cache 222,providing data to L1 I-Cache 221 and L1 D-Cache 222. L2 Cache 223obtains data from a lower level cache (not shown) or from main memorythrough memory interface 225.

Caches at any level are logically extensions of main memory. In theexemplary embodiment, L1 and L2 caches 221-223 are physically packagedwith the CPU, e.g., are implemented on the same integrated circuit chipas the CPU. For this reason, these caches are sometimes considered apart of the CPU. In this embodiment, each CPU has its own respective L1and L2 caches, which are not shared with other CPUs, although it isalternatively possible that some or all caches be shared. Therepresentation of FIG. 2 is intended to be typical, and is not intendedto limit the present invention to any particular physical or logicalcache implementation. It will be recognized that processors and cachescould be designed according to different arrangements, and the processorchip or chips may include more caches or fewer caches than representedin FIG. 2, or no caches at all.

Instruction unit 201 comprises branch unit 202, instructiondecode/dispatch unit 203, and instruction registers and buffers 204.Instructions from L1 I-cache 221 are loaded into buffers 204 prior toexecution. Depending on the CPU design, there may be multiple buffers(e.g., buffers for different threads, or within a thread, one for asequential series of instructions, and others for branch-to locations),each of which may contain multiple instructions. Decode/dispatch unit203 selects one or more instructions to be dispatched for execution fromone or more of buffers 204 in a current machine cycle, and decodes theinstruction(s) according to the semantics of the processor's pre-definedinstruction set to determine the operation(s) to be performed or branchconditions. Branch unit 202 controls the program flow by evaluatingbranch conditions, and refills buffers 204 from L1 I-cache 221.

Execution unit 211 comprises a set of general purpose registers 212 forstoring data and a scalar arithmetic logic unit (ALU) 213 for performingarithmetic and logical operations on data in general purpose (GP)registers 212 responsive to instructions decoded by instruction unit201. Execution unit 211 may further include any of variousspecial-purpose computational subunits. For example, depicted in FIG. 2is a floating point operations subunit 214, which is preferably aspecial floating point hardware pipeline for performing floating pointoperations using larger operands (e.g. double-precision 64-bitoperands). Other special purpose units (not shown) could optionally beincluded, such as a vector operation subunit for performing commonoperations on multiple operands in parallel. Floating point subunit 214(and optionally, other special purpose units) each includes its ownrespective set of registers 215. Both general-purpose registers 212 andfloating point registers 215 are generally available for use by compiledprograms executing on processor 101. In particular, a compiler compilingprograms for execution on processor 101 can use any or all of theseregisters for holding arbitrary program state data as the compilerdetermines will result in most efficient execution of the program. Inaddition to components shown in FIG. 2, execution unit 211 may includeadditional logic, counters, control hardware, and so forth, and couldinclude multiple copies of some units, such as multiple ALUs 213. Itwill be understood that the execution unit 211 represented in FIG. 2 isintended to be representative, and that an execution unit may haveadditional subunits and components (including additional pipelines andregisters) or may have fewer than all of the components shown in FIG. 2.

Special registers 205 contain certain state data other than instructions(contained in instruction registers 204) and general purpose data uponwhich instructions operate (contained in registers 212, 215). Forexample, special registers 205 may include machine state register 206which may contain, among other things, data indicating a privilege levelof a currently executing thread or threads (if the CPU supports multipleconcurrent thread execution); interrupt vectors 207; error indications;and other special registers. In particular, special registers 205include one or more context switch registers 208, which are reserved foruse by a specially privileged process, such as an operating systemkernel, for holding certain state information used in switchingexecution contexts. I.e., when an executing process thread is suspendedfor any of various reasons, to be later resumed, the thread state, andin particular the state of the registers, is saved in a context savearea in memory 102 before another thread's executable instructions andstate data are loaded and begin execution on the processor. Theoperating system uses context switch registers 208 for holding dataneeded for context switching, in particular, address data identifyingthe location of the context save area in memory.

L1 I-cache 221 and L1 D-cache 222 are separate instruction and datacaches providing data to instruction and execution units, although theymay alternatively be a single combined cache. L2 cache 223 is anon-discriminated cache containing both instructions and non-instructiondata. Typically, data is taken from or stored to an L1 cache by theinstruction or execution unit, and if the data is unavailable in an L1cache, it is loaded into the L1 cache from L2 cache 223, which in turnobtains it from a lower level cache or main memory through memoryinterface 225. Depending on the processor design, it may be possible toby-pass a cache at one level and load data from a lower level cache ormemory.

Address translation unit 224 translates effective addresses (in somearchitectures, called “virtual addresses” or some other name) generatedby the processor to corresponding real addresses in memory. As is knownin the art, a fundamental difference exists between effective addresseson the one hand, and real addresses on the other. An effective addresshas no fixed correspondence to a physical memory location; thiscorrespondence changes as new pages are loaded into main memory fromstorage, as processes change, and so forth. A real address correspondsto a fixed physical memory location, although it does not necessarilytranslate directly to the location. The processor generates effectiveaddresses (which may be called “virtual” or some other term) in aneffective address space corresponding to each respective executingprocess. In some computer architectures, there are multiple levels ofeffective or virtual address, which may require further translation. Aprocessor-generated effective address could, in some architectures,simply be an address contained in the instruction itself. However, inmost modern systems, the effective address space is so large relative tothe size of the instruction that instructions do not contain fulladdresses. Therefore, an address is contained in a register which isreferenced by the instruction, or obtained as a sum of multiple values,such of an offset value contained in the instruction and one or morevalues each contained in a respective register. The effective addressesare further translated to “real addresses”, corresponding to the actualmemory locations at which the data is located by address translationunit 224. It will be understood that various computer architecturesemploy different addressing constructs, and the present invention is notlimited to any particular form of addressing.

L1 I-Cache 221 and L1 D-Cache 222 are preferably addressed usingeffective addresses, and therefore no address translation is requiredfor accessing the level 1 caches. However, the L2 Cache 223 and allmemory below it are addressed using real addresses. Therefore, if it isnecessary to access a lower level cache or main memory, an effectiveaddress generated by the processor is first translated to a realaddress. It will be understood that the L1 caches could alternatively beaddressed with real addresses, or any of the lower level caches couldalternatively be addressed with effective addresses.

Address translation unit 224 is represented as a single logical entity,but typically includes multiple tables and logic circuitry, which may bedistributed in various chip locations. For example, an addresstranslation mechanism may include a translation look-aside buffer, aneffective-to-real address translation table, a segment table, andadditional structures. Additionally, separate structures could be usedfor translation of instructions and for non-instruction data.

CPU 101 may be a multithreaded processor supporting the concurrentexecution of multiple threads and simultaneous dispatching ofinstructions from different threads in the same machine cycle, or it maybe a single threaded processor. Where multi-threading is supported, aseparate set of most registers typically exists for each thread. I.e., aseparate set of general purpose registers 212 and floating pointregisters 215 exists for each thread. Additionally, certain other stateor special purpose registers may be duplicated to support multipleactive threads. The execution unit pipeline hardware, the instructionunit, and the caches are typically shared by all threads.

While various CPU components have been described and shown at a highlevel, it should be understood that the CPU of the preferred embodimentcontains many other components not shown, which are not essential to anunderstanding of the present invention. For example, various additionalspecial purpose registers will be required in a typical design.Furthermore, it will be understood that the CPU of FIG. 2 is simply oneexample of a CPU architecture, and that many variations could exist inthe number, type and arrangement of components within CPU 101, thatcomponents not shown may exist in addition to those depicted, and thatnot all components depicted might be present in a CPU design. Forexample, the number and configuration of buffers and caches may vary;the number and function of execution unit pipelines may vary; registersmay be configured in different arrays and sets; dedicated floating pointhardware may or may not be present; etc. Furthermore, CPU 101 mayutilize a simple or complex instruction set.

FIG. 3 is a conceptual illustration of the major software components ofsystem 100 in memory 102. Operating system kernel 301 is executable codeand state data providing various low-level software functions, such asdevice interfaces, management of memory pages, management anddispatching of multiple tasks, etc. as is well-known in the art. DynamicBinary Optimizer 302, sometimes also called a virtual machine, is anexecutable program for supporting execution and dynamic optimization ofa compiled executable target program, as explained in greater detailherein. Static compiler 303 is an executable computer program whichcompiles source code modules in a high-level language (or,alternatively, code modules in an intermediate, previously parsedsymbolic form) into object code modules of processor-executableinstructions. Build utility 304 is an executable computer program whichbuilds application programs by incorporating or linking (also known asbinding) multiple previously compiled object code modules and programs.

Also shown in FIG. 3 are multiple source code modules 311A-C (hereingenerically referred to as feature 311, multiple intermediate codemodules 312A-C (herein generically referred to as feature 312), andmultiple object code modules 313A-C (herein generically referred to asfeature 313). Source code modules 311 are code modules in a high levellanguage created using a source editor (not shown) or general-purposeeditor (not shown). Such modules could be created on another computersystem. Intermediate code modules are code modules in an intermediatesymbolic language, which is neither a high-level language intended forhuman representation, nor directly executable code. Examples ofintermediate code representations are Stanford p-code, IBM™ w-code, IBM™New Machine Interface (NMI) code, and JAVA™ bytecodes. Intermediate codeis produced from the source by a special purpose parser or “front-endcompiler” (not shown), or alternatively as a by-product of compilationby static compiler 303. Object code modules 313 are modules containingprocessor-executable instructions which are directly executable onprocessor 101, and are produced by static compiler 303. Each object codemodule is produced either directly from a corresponding source module311, or by pre-compiling a source module to an intermediate code module312, and then compiling the intermediate code module to a correspondingobject code module.

User application programs 314, 315 are executable programs which arebuilt by build utility 304 from multiple object code modules 313. Thebuild utility may further incorporate compiled procedures from variousprogram libraries (not shown) when building an application program.

OS Kernel 301 supports concurrent execution of multiple processes, as iswell known in the art. For each concurrently executing process, arespective process state data area 321-323 is maintained, each processstate data area containing state data particular to the executingprocess. FIG. 3 depicts three process state data areas 321-323, it beingunderstood that this number may vary, and is typically much larger. Eachprocess state data may include, e.g., a process stack, a process heap,and/or other data structures.

In particular, a process executing a target user application programthrough an instance of dynamic binary optimizer 302 is represented asprocess state data area 323. In addition to any of the various datastructures which may be present in any process state data area, processstate data area 323 includes a target program context save area 324, atarget program optimized code cache 325, and a target program data area326. Target program context save area 324 is used to temporarily storethe processor state when the target program is interrupted by theoptimizer so that the optimizer can perform various optimizationfunctions. Target program optimized code cache 325 stores target programcode as altered by the optimizer during dynamic optimization; thisoptimized code is not necessarily saved, and the original target programexecutable code remains unchanged as a result of executing the targetprogram with the dynamic binary optimizer. Target program data area 326holds data referenced by the target program during execution.

Various software entities are represented in FIG. 3 as being separateentities or contained within other entities. However, it will beunderstood that this representation is for illustrative purposes only,and that particular modules or data entities could be separate entities,or part of a common module or package of modules. Furthermore, althougha certain number and type of software entities are shown in theconceptual representation of FIG. 3, it will be understood that theactual number of such entities may vary, and in particular, that in acomplex computer system environment, the number and complexity of suchentities is typically much larger, and that other entities (not shown)may be present. Additionally, although software components 301-304,311-315, and 321-326 are depicted in FIG. 3 on a single computer system100 for completeness of the representation, it is not necessarily truethat all programs, functions and data will be present on a singlecomputer system or will be performed on a single computer system. Inparticular, static compiler 303 and/or build utility 304 may be locatedon a different system than dynamic binary optimizer 302, and any ofsource code 311, intermediate code 312, or object code modules 313 mayor may not be present in the same system as the dynamic binaryoptimizer. In fact, although source modules are illustrated in FIG. 3for completeness, in many cases source modules may no longer exist or,if they exist, may be unavailable or in an unknown location.

While the software components of FIG. 3 are shown conceptually asresiding in memory 102, it will be understood that in general the memoryof a computer system will be too small to hold all programs and datasimultaneously, and that information is typically stored in data storagedevices 125-127, comprising one or more mass storage devices such asrotating magnetic disk drives, and that the information is paged intomemory by the operating system as required. Furthermore, it will beunderstood that the conceptual representation of FIG. 3 is not meant toimply any particular memory organizational model, and that system 100might employ a single address space virtual memory, or might employmultiple virtual address spaces that overlap.

FIG. 4. is a high-level flow diagram showing a general process ofcreating an executable target program and executing the target programusing a dynamically binary optimizer, in accordance with the preferredembodiment. Referring to FIG. 4, a program can either be compileddirectly from the source code, as indicated by the flow starting fromblock 401, or may be compiled from some intermediate form of code, asindicated by the flow starting from block 402. Source code as indicatedby block 401 may be generated in any conventional manner by theprogrammer, such as with a source editor. In accordance with thepreferred embodiment, no special statements or structure is required ofthe source code, and in general any conventional source code in ahigh-level language can be dynamically optimized, although not all codewill optimize with the same efficiency. An intermediate form of code maybe generated by a special intermediate code generator for that purpose,or may be a by-product of previous compilation. For example, in the caseof the IBM i platform, the compiler automatically generates anintermediate representation known as “NMI code” and includes it in thecompiled program object along with the processor-executableinstructions. Often, in the case of legacy programs, the original sourcecode may no longer be available. However, if the original source wascompiled on a platform or using a compiler which generates anintermediate representation, such as “NMI code”, then the program can berecompiled from the intermediate representation, notwithstanding thatthe original source is not available. The source or the intermediatecode representation could have been generated many years beforecompilation and execution as described in blocks 403-405, and could havebeen generated on a different system using a different processor and/oroperating system architecture.

Whether using source code or an intermediate representation, a compilercompiles one or more program modules to produce modules ofprocessor-executable instructions (also known as object code modules),in which one or more registers are reserved for use by a dynamic binaryoptimizer. This compilation process is represented at a high level asblock 403 in FIG. 4, and is shown in greater detail in FIG. 5.

FIG. 5 is a high-level flow diagram showing a process of staticallycompiling one or more modules of the target program for execution usinga dynamic binary optimizer, in accordance with the preferred embodiment.

Referring to FIG. 5, static compiler 303 is invoked and initialized(block 501). A source code file 311 or intermediate code file 312 to becompiled is specified when the compiler is invoked.

Among various other parameters, the static compiler implicitly orexplicitly determines a number of available general purpose registers212 for use by the target program, designated N_(GP) (block 502). Thisnumber could be hard-coded in the compiler, but is more likely receivedas an input parameter to the compiler. For example, the compiler might,through an operating system call, determine a type of processor andnumber of general purpose registers in the processor(s) of the computersystem on which it is executing, and use this number as a default. Thisdefault might be overridden by an explicit compiler directive to compilethe code for use in a different machine having a different processortype.

The compiler further determines whether it should reserve any generalpurpose registers for use by a dynamic binary optimizer in addressing acontext save area (block 503). Preferably, reservation of one or moreregisters is performed responsive to a compiler directive. The compilerdirective could be a statement in source code file 311 or intermediatecode file 312, but in the preferred embodiment it is an optionalcompiler parameter which is specified by the user at the time thecompiler is invoked. It is preferable to avoid requiring that such adirective be embedded in the source or intermediate code file, sincesuch files could be very old, and it may difficult to insert statementsinto them if editing tools or front-end compilers/parsers are no longeravailable. It would alternatively be possible for reservation of one ormore registers for the dynamic binary optimizer to be a default option,which is automatically selected unless the user directs the compiler notto reserve a register. Alternatively, the compiler might be written toalways reserve at least one register, although this is consideredundesirable because there may be circumstances where it is desirable touse all registers.

If the compiler determines that is should reserve a register (the ‘Y’branch from block 503), the number N_(GP) of available general purposeregisters is decremented by the number of registers to be reserved,N_(R) (block 504). In the preferred embodiment, N_(R) is one, although alarger number could alternatively be used or specified by the user.Since each reserved register necessarily reduces the number of registersavailable to the compiler for assigning variables, and therefore tendsto increase the need to swap values in and out of registers and therebyreduce execution performance, it is undesirable to reserve any moreregisters than necessary. A single register will generally be sufficientto specify the context save area address, and therefore N_(R) ispreferably one. Depending on the architecture, it may be desirable toreserve one or more additional registers for related context saveoperations. If the compiler determines at block 503 that no generalpurpose registers should be reserved, then block 504 is by-passed, andthe value of N_(GP) remains unchanged.

The compiler optionally parses source file 311 to generate anintermediate symbolic code representation and a mapping of variables,procedure names, and so forth used in the source (block 505). Thisprocess is sometimes referred to as “front-end compilation”. Generally,this is only necessary if compiling directly from source code. If thecompiler is compiling from an intermediate code version, theintermediate code will typically contain the necessary information.

The compiler constructs a control flow graph (block 506). As is known inthe compiler art, a control flow graph is a representation of flow ofcontrol in the code module being compiled, containing multiple nodes anddirected arcs connecting the nodes, where each node represents asequence of code instructions having only a single straight path ofexecution, and each arc represents a possible path (such as a branch)from one node to another.

Using the control flow graph, the compiler chooses an assignment ofprogram variables to available registers (block 507), the number ofgeneral purpose registers available for this purpose being N_(GP). Anyof various register assignment techniques, now known or hereafterdeveloped in the compiler art, may be employed, so long as the number ofavailable general purpose registers is limited to N_(GP). Depending onthe needs of the program and the assignment technique used, the compilermay, although it will not necessarily, assign all of the availablegeneral purpose registers for use by one or more program variables, butin any case, if one or more general purpose registers was reserved atblocks 503-504, those reserved registers will not be assigned.

The compiler performs any supported code optimizations and generatesoptimized, compiled code, using the register assignments made previously(block 508). The optimizations performed will depend on the compiler andcould, include, for example, removal of unnecessary code statements,re-ordering of operations, combining of aliases, and so forth, as isknown in the art. It is not essential that the compiler perform anyparticular optimization, or that the compiler be an optimizing compiler.Although FIG. 5 depicts optimizations (block 508) being performed afterregister assignment (block 507), register assignment could be performedbefore or after optimizations, or before some optimizations and afterothers; typically at least some optimizations are performed beforeregister assignment.

The compiler typically is invoked separately for each code module to becompiled. Different code modules may be compiled at approximately thesame time, or may be compiled at different times, and may be compiled ondifferent machines, or on a single machine different from that on whichthey are to be executed.

Referring again to FIG. 4, after compilation, a program is built fromone or more compiled object code modules (block 404) using build utility304. Build is a process of linking multiple compiled object code modulesinto a single executable program. The resultant executable program may,e.g., be a single file which contains a header and embeds code from oneor more compiled object code modules, and may contain references toexternal object code modules. Depending on the architecture and/or thesize of the program, a build may be unnecessary and it is possible thatcompilation alone produces a program in executable form.

The executable program is generally a file (or multiple files). As aprogram file, it may be stored on the system indefinitely and/or loadedto another computer system. In some architectures, build is generallyperformed immediately prior to execution.

The program is then executed using the dynamic binary optimizer. Programexecution is represented at a high level as block 405 in FIG. 4, and isshown in greater detail in FIG. 6.

FIG. 6 is a high-level flow diagram showing a process of executing atarget program using dynamic binary optimizer 302, in accordance withthe preferred embodiment. Referring to FIG. 6, the dynamic binaryoptimizer is invoked for execution of the target program and allocatesstate data structures in the applicable process data area (block 601).The dynamic binary optimizer is itself an executable program. Whenexecuted, it emulates a virtual machine which executes other executableprograms, in this case the target program, and thus the target programis a program within a program Like any executable program, the dynamicbinary optimizer is invoked on behalf of a user process, the userprocess having a process data area. A process data area for the processexecuting the dynamic binary optimizer is shown in FIG. 2 as feature323. Among the state data which the dynamic binary optimizer maintainsis target program optimized code 325, target program data 326, andtarget program context save area 324.

Dynamic binary optimizer 302 inserts traps into various compiled targetprogram modules (block 602). Each trap causes execution of the targetprogram's code sequence to halt and returns control to the dynamicbinary optimizer. A trap might be nothing more than a singleprocessor-executable instruction branching to a pre-determined codesequence of the optimizer itself. Traps are inserted at convenientlocations for tracking target program execution flow and determiningwhether to optimize selective code portions. For example, a trap mightbe inserted every time there is a call to or return from a procedure.

After initializing and inserting any necessary traps, the dynamic binaryoptimizer jumps to an entry point in the compiled target program (block603), causing a segment of the processor-executable instructions of thetarget program to execute directly on the processor 101 (block 604).Although the word “segment” is used, the instructions which are executedare not necessarily contiguous in addressable memory, nor is the segmentnecessarily of any pre-determined size. It is simply a portion of theprogram having one or more exit points and corresponding traps insertedat each exit point. During execution of the segment, the segment'sprocessor-executable instructions may reference data in the targetprogram data area 326, loading and storing data from this area of memoryin the same manner as if it were executing directly, without anyintervention of the dynamic binary optimizer. The program segmentcontinues to execute until a trap is encountered (block 605).

Preferably, the trap causes a jump to a pre-determined code segmentwithin the dynamic binary optimizer itself. The first order of businessupon trapping is to save the state of the processor (block 606). Forthis purpose, the reserved general purpose register (GP Register R) isused to address target program context save area 324. For example, thefirst instruction encountered upon jumping to the trap location might bea store of the contents of GP Register 0 to the effective memory addressdesignated by GP Register R; a second instruction might be a store ofthe contents of GP Register 1 to the effective memory address designatedby the sum of the value in GP Register R and a specified offset, and soforth. As the contents of the various general purpose registers arestored, these registers become available for use by the dynamic binaryoptimizer in saving other state data, which may require more than asingle register. Thus, as described only a single reserved register isneeded for saving the processor's state.

Once the target program's processor state has been saved, the dynamicbinary optimizer can perform any desired optimization functions. Theoptimizer determines whether execution of the target program hascompleted (block 607). If not (the ‘N’ branch from block 607), theoptimizer updates any execution data used for performing optimizations(block 608). For example, an optimizer will typically track thefrequency of execution of selective code segments, and re-optimize andre-compile code if the segment is frequently executed. The optimizer mayalso choose to re-compile code deemed particularly inefficient. Theoptimizer therefore uses this execution data to determine whether a codesegment (such as a procedure) should be re-optimized (block 609). If thedecision is made to re-optimize a code segment, the corresponding codesegment is re-optimized and re-compiled (block 610), and stored intarget program optimized code area 325. Often, a segment to bere-optimized is one which would otherwise be executed next.

In the preferred embodiment, among the optimizations which may beperformed by the dynamic binary optimizer are optimizations whichutilize aliasing information, and in particular optimizations whichinvolve re-ordering of execution of certain operations which aliasinginformation indicates may be safely re-ordered. Aliasing information ispreferably embedded in object code modules 313 at compile time for thispurpose. This technique is described in greater detail in commonlyassigned U.S. patent application Ser. No. 13/016,038, filed on the samedate as the present application, entitled “Using Aliasing Informationfor Dynamic Binary Optimization” (Assignee's docket no.ROC920100267US1), which is herein incorporated by reference. However, inan alternative embodiment, such aliasing information is not presentand/or is not used for optimization.

When the optimizer has completed updating its data and performed anydesired optimizations, it reloads the processor state from the targetprogram context save area and resets the value of GP Register R to theaddress of the beginning of the context save area (block 611). It thenjumps to the code location at which target program execution wassuspended (block 612), and the next target program segment executes(block 604).

When the target program has finished execution (i.e, it encounters atrap at an exit point in the program), the ‘Y’ branch is taken fromblock 607. The optimizer then performs any required final clean-up ofdata structures, output of data, etc., and ends execution (block 613).

As described above, multiple traps are inserted at block 602 at thebeginning of execution. However, the optimizer might alternativelyinsert one or more traps in each target program segment at each exitpoint thereof immediately before the segment is to be executed (if notpreviously inserted), thereby avoiding the need to insert traps inprogram segments which are never executed. Furthermore, althoughinserting traps is disclosed herein as an exemplary embodiment, thereare alternative techniques by which a dynamic binary optimizer may gaincontrol of the processor. For example, some systems use periodicinterrupts to sample the program counter and decide when to optimize asegment of code. Alternatively, there may be hardware assists to helpgather branch traces and invoke the optimizer.

Typically, the dynamic binary optimizer does not generate any persistentoptimized code which survives the user process which caused it to begenerated. Depending on the implementation of the dynamic binaryoptimizer, it is possible for a single user process to execute theprogram multiple times and to re-use the optimized code generated duringa previous execution. However, once the user process dies, theoptimizations are not saved. This is the nature of dynamic binaryoptimization. If the previously compiled program is subsequentlyexecuted again using the dynamic binary optimizer in a different userprocess, the optimizer will typically begin anew with the program asstatically compiled at block 403 and built at block 404, without thebenefit of any previous optimizations made in a previous user process.

Although a specific sequence of operations is illustrated in the flowdiagrams and described in the accompanying text, it will be appreciatedthat some operations could be performed in a different order, that someoperations need not be performed, and that other operations may beperformed instead, consistent with the present invention.

In general, the routines executed to implement the illustratedembodiments of the invention, whether implemented as part of anoperating system or a specific application, program, object, module orsequence of instructions, including a module within a special devicesuch as a service processor, are referred to herein as “programs” or“control programs”. The programs typically comprise instructions which,when read and executed by one or more processors in the devices orsystems in a computer system consistent with the invention, cause thosedevices or systems to perform the steps necessary to execute steps orgenerate elements embodying the various aspects of the presentinvention. Moreover, while the invention has and hereinafter will bedescribed in the context of fully functioning computer systems, thevarious embodiments of the invention are capable of being distributed asa program product embodied in non-transitory computer-readable media,and the invention applies equally regardless of the form ofdistribution. Examples of non-transitory computer-readable mediainclude, but are not limited to, volatile and non-volatile memorydevices, floppy disks, hard-disk drives, CD-ROM's, DVD's, and magnetictape, it being understood that these examples are not exhaustive.Examples of non-transitory computer-readable media are illustrated inFIG. 1 as system memory 102 and data storage devices 125-127.

Unless inconsistent with the invention or otherwise qualified herein,computer program code for carrying out operations of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The program code may execute entirely on the user's computer,partly on the user's computer, as a stand-alone software package, partlyon the user's computer and partly on a remote computer or entirely onthe remote computer or server. In the latter scenario, the remotecomputer may be connected to the user's computer through any type ofnetwork, including a local area network (LAN) or a wide area network(WAN), or the connection may be made to an external computer (forexample, through the Internet using an Internet Service Provider).

The present invention is described herein with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products according to embodiments of the invention. Itwill be understood that each block of the flowchart illustrations and/orblock diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks.

These computer program instructions may also be stored in anon-transitory computer-readable medium that can direct a computer orother programmable data processing apparatus to function in a particularmanner, such that the instructions stored in the non-transitorycomputer-readable medium produce an article of manufacture includinginstruction means which implement the function/act specified in theflowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide processes for implementing the functions/actsspecified in the flowchart and/or block diagram block or blocks.

The flowcharts and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

Although a specific embodiment of the invention has been disclosed alongwith certain alternatives, it will be recognized by those skilled in theart that additional variations in form and detail may be made within thescope of the following claims:

What is claimed is:
 1. A computer-implemented method for executing atarget program in a computer system, comprising: compiling code of saidtarget program for execution on a computer system processor to producean executable target program having original executable code, saidprocessor having a set of N registers available for use by programsexecuted by the processor, wherein N≧2, each register of said set of Nregisters being assignable during compilation of said programs toprogram variables used by said programs; reserving, during saidcompiling code of said target program, at least one register of said setof N registers for later use by a dynamic binary optimizer duringexecution of said target program, wherein the number of registers ofsaid set of N registers assignable by compiling code of said targetprogram to program variables used by said target program is no more thanN less the number of registers of said set of N registers which arereserved; and executing said executable target program with a dynamicbinary optimizer which generates dynamically optimized executable codereplacing said original executable code of at least one portion of saidtarget program, said dynamic binary optimizer using said at least oneregister of said set of N registers which is reserved during executionof said target program to hold address information for a context savearea to which register contents are saved when said dynamic binaryoptimizer switches execution between execution of said originalexecutable code of said target program and said dynamically optimizedexecutable code of said target program.
 2. The method of claim 1,wherein compiling code of said target program comprises compiling froman intermediate code representation.
 3. The method of claim 1, whereincompiling code of said target program comprises compiling from a sourcecode representation.
 4. The method of claim 1, wherein compiling code ofsaid target program is performed by a compiler having at least twoselectable compilation options, including a first compilation optionwherein all N registers of said set of N registers are assignable bysaid compiler to program variables used by a program being compiled bysaid compiler, and a second compilation option wherein at least oneregister of said set of N registers is reserved and not assignable bysaid compiler to program variables used by a program being compiled bysaid compiler.
 5. The method of claim 1, wherein executing saidexecutable target program with a dynamic binary optimizer comprisesinserting a plurality of traps in said target program, each trapencountered during execution of said target program causing a respectivecontext switch to said dynamic binary optimizer.
 6. The method of claim1, wherein executing said executable target program with a dynamicbinary optimizer comprises: (a) executing instructions of said targetprogram on said computer system processor until the occurrence of acontext switch event; (b) upon the occurrence of a context switch event,executing instructions of said dynamic binary optimizer on said computersystem processor to save processor state of said target program in acontext save area in memory; (c) thereafter executing instructions ofsaid dynamic binary optimizer on said computer system processor toperform at least one other function of said dynamic binary optimizer;(d) thereafter executing instructions of said dynamic binary optimizeron said computer system processor to restore said processor state ofsaid target program from said context save area, and resuming executionof instructions of said target program on said computer systemprocessor; and (e) repeating (a) through (d) until completion ofexecution of said target program.
 7. The method of claim 6, whereinaddress information for said context save area is stored in said atleast one register of said set of N registers which is reserved during(a), and is used during (b) to identify a destination address for savingsaid processor state of said target program.
 8. A computer programproduct for compiling a target program for execution on a computersystem, comprising: a non-transitory computer-readable medium havingcomputer usable program code embodied therewith, the computer usableprogram code being configured to: receive a target program to becompiled for execution on a computer system processor, said processorhaving a set of N registers available for use by programs executed bythe processor, wherein N≧2, each register of said set of N registersbeing assignable during compilation of said programs to programvariables used by said programs; receive a register reservation optionselection, said register reservation option selection selecting fromamong a plurality of register reservation options, including a firstregister reservation option wherein all N registers of said set of Nregisters are assignable by said computer program product to programvariables used by said target program, and a second register reservationoption wherein at least one register of said set of N registers isreserved for use by a dynamic binary optimizer which generatesdynamically optimized processor-executable code of at least one portionof said target program replacing compiled processor-executable codegenerated by said computer program product during execution of saidtarget program and not assignable by said computer program product toprogram variables used by said target program; responsive to receivingsaid first register reservation option, assign program variables of saidtarget program to all N registers of said set of N registers; responsiveto receiving said second register reservation option, assign programvariables of said target program to a number of registers of said set ofN registers equal to N less the number of registers reserved by saidsecond reservation option; generate compiled processor-executable codeof said target program using the register assignments made responsive tosaid register reservation option selection; wherein said dynamic binaryoptimizer uses said at least one register of said set of N registers tohold address information for a context save area to which registercontents are saved when said dynamic binary optimizer switches executionbetween execution of said compiled processor-executable code generatedby said computer program product and said dynamically optimizedprocessor-executable code of said target program.
 9. The computerprogram product of claim 8, wherein said computer usable program code ofsaid computer program product is executable on said processor.
 10. Thecomputer program product of claim 8, wherein the target program to becompiled received by said computer program product is an intermediatecode representation.
 11. The computer program product of claim 8,wherein the target program to be compiled received by said computerprogram product is a source code representation.
 12. A digital dataprocessing system, comprising: a memory; a processor executing programscontaining instructions storable in said memory, said processor having aset of N registers available for use by said programs wherein N≧2, eachregister of said set of N registers being assignable during compilationof each program of said programs to program variables used by theprogram; a dynamic binary optimizer embodied as instructions executableon said processor, said dynamic binary optimizer for executing targetprograms of said programs, said dynamic binary optimizer generatingrespective dynamically optimized code executable on said processor of atleast a portion of each target program which replaces compiled code ofeach target program executable on said processor; a compiler forcompiling target programs for execution using said dynamic binaryoptimizer, said compiler reserving, during compilation of each targetprogram, at least one register of said set of N registers for later useby said dynamic binary optimizer during execution of the target program,wherein said dynamic binary optimizer uses said at least one register ofsaid set of N registers to hold address information for a context savearea to which register contents are saved when said dynamic binaryoptimizer switches execution between execution of compiled codegenerated by said compiler and said dynamically optimized code of thetarget program, and wherein the number of registers of said set of Nregisters assignable by said compiler to program variables used by saidtarget program is no more than N less the number of registers of saidset of N registers which are reserved; and at least one target programcompiled by said compiler.
 13. The digital data processing system ofclaim 12, wherein said compiler is embodied as instructions executableon said processor.
 14. The digital data processing system method ofclaim 12, wherein said compiler has at least two selectable compilationoptions, including a first compilation option wherein all N registers ofsaid set of N registers are assignable by said compiler to programvariables used by a program being compiled by said compiler, and asecond compilation option wherein at least one register of said set of Nregisters is reserved and not assignable by said compiler to programvariables used by a program being compiled by said compiler.
 15. Thedigital data processing system of claim 12, wherein said dynamic binaryoptimizer executes a target program by: (a) executing instructions ofsaid target program on said processor until the occurrence of a contextswitch event; (b) upon the occurrence of a context switch event,executing instructions of said dynamic binary optimizer on saidprocessor to save processor state of said target program in a contextsave area in said memory; (c) thereafter executing instructions of saiddynamic binary optimizer on said processor to perform at least one otherfunction of said dynamic binary optimizer; (d) thereafter executinginstructions of said dynamic binary optimizer on said processor torestore said processor state of said target program from said contextsave area, and resuming execution of instructions of said target programon said processor; and (e) repeating (a) through (d) until completion ofexecution of said target program.